My wiring does not correspond with the schematics in the Chilton Manual for my truck. My '90 GMC 1500 C originally came with a V6 and standard tranny but they were swapped out by a previous owner.

I assume that the harness and engine came from the same vehicle. Where the harness go through the firewall on the passenger side it was originally through a hole of about 2.5" X 2.5" but the grommet on the wiring harness was about 2.5" X 4", see pic below.

Any ideas how to identify the wiring harness? I tried to ID the engine but it appears to be a crate engine as it does not have the complete block code.

I have a electrical drain that is difficult and so a schematic will help a lot. I am in the Desert and am not to happy about long hours tracing wires in three digit heat without a schematic.

 

Hmm .. unfortunately, I really don't know how to ID that harness. I'd say that it will be EXTREMELY difficult to figure out unless you can find out what vehicle that harness came from .. even different years can change the harness somewhat. Or an A/C equipped truck versus a non A/C work truck etc. Do you know what circuit is causing the drain? Put your amp meter between the negative battery cable and the battery post, and watch the meter. Pull fuses until you see your parasitic draw drop (with the key off, door switch taped closed so the lights don't come on etc). Be careful with this ... something you do all the time, such as opening your door with your meter connected can blow your fuse. So I'd suggest taping the door switch closed so the lights stay off, key off, and unplug your under hood light.

 

Yea! I did the fuse thing, I also pulled the dash off and unplugged everything. When I unplug the harness going in to the dash on the passenger side and the harness coming out on the driver side the drain stops. And the alternator unplugged.

There is a drain when I plug either of the plugs going into the Control Module only or the fat red wire alone all of which come in to cable on passenger side.

If it was cooler I'd actually enjoy the puzzle but I want to have the truck running so I can move out of state at the end of the month.

OK! I did something really dumb I put the battery cables on wrong for about 30 seconds but I am thinking I that had the drain before that but am not sure. Anyway I have the drain and and have to find it.

I am sure I fried a diode in the alternator but that easy, I have it unplugged.

 

Okay so you figured out what circuits are causing the drain? Sounds like your alternator is causing the drain?

 

No! there is still a drain with the alternator unplugged. The harness and engine are from a different year and or model and I think that there are some adjustments that has to be made to make it compatible with the truck stuff.

If I had both schematics I can make it work, well, I can make it work anyway but it sure would be better with. I am going to go out later with labels and markers and make some head way getting familiar with everything and see what I see.

 

Well good luck! I'd say leave your engine harness and everything plugged in and then try the fuse method. Unplug one thing at a time until you figure out what component or circuit is creating the parasitic drain.

 

As I said, "I did the fuse thing" first, then I started to unplug things. I wonder it it is that the harness is wrong or a circuit is grounded in the Control Module?

The engine still starts.

 

Don't forget about possible parallel path(s). Just a thought.
Good Luck.
GD

 

I understand Parallel Paths in the generic sense but can you expound please?
This is a similar wire diagram.

There is one thing that I found that consern me, there is a Tan wire that come from the Control Mudule that goes to ground on the engine block, also there is a Black wire with White stripe doing like wise. I find Tan to be an odd color for a ground wire. I ran outta sweat and stopped before I could check the drain with the Tan disconnected.

 

The main point is that I have to get a proper wiring diagram and don't know how to ID the one I got to be able to find the right one.

I am thinking that if I can know what years certain components were available on the engine I can narrow it down. The Chillton manual I have is for '88 to '98 but I expect there wiring diagrams to be not so accurate.